1. Field of the Invention
The present invention relates to a zoom lens, and to an imaging device, a camera device and a mobile information terminal using the zoom lens as a photographing optical system.
2. Description of the Related Art
In recent years, a type of camera known as a digital camera or an electronic camera has become popular. In such a type of camera, an image of a subject is captured by a solid-state image sensor including an image pickup device or the like which includes for example a CCD (charge-coupled device), C-MOS image sensor or the like, image data of a still image or a moving image of the subject is obtained, and then the data is recorded digitally in a nonvolatile semiconductor memory represented by a flash memory. On the other hand, a conventional camera using a traditional silver salt film, i.e. a silver salt camera, is gradually becoming a thing of the past.
The market for such a digital camera is huge, and the demand of users for a digital camera varies over a wide range. In particular, a user always requires high image quality, wide field of view, and a reduction of size of the camera.
A variable focal length lens such as a zoom lens or the like is often used as a photographing lens in this type of digital camera, to utilize the features of compactness, lightness, and high performance. In most cases, a zoom lens including two groups or three groups of lenses which is not constituted of many lenses is used as the zoom lens for the digital camera.
In addition, high image quality compatible with a light-receiving element having more than three million pixels, as well as a wide field of view, large diameter and compactness are required in the zoom lens.
A conventional imaging device which is suitable for a digital camera or the like is disclosed in the following reference documents, by way of example: JP 2003-131134A, JP 2003-107352A, JP 2003-35868A, and JP 2005-24988A. Each of these conventional imaging devices is suitable for downsizing, has a wide field of view of 38 degrees or more in a half-field angle, and has an F-number of three or less in a short focal length end,
Namely, the above-mentioned reference documents each discloses an imaging device using a zoom lens which has a wide field of view of 38 degrees or more in a half-field angle and has an F-number of three or less in a short focal length end, wherein the zoom lens used in the imaging device includes a first group optical system having a negative focal length; a second group optical system having a positive focal length; a third group optical system having a positive focal length, the first group optical system, the second group optical system and the third group optical system being arranged in this order from an object side to an image side; and an aperture stop configured to move together with the second group optical system and disposed on the object side of the second group optical system, a change in magnification being carried out by decreasing a space between the first group optical system and the second group optical system when the change in magnification is carried out from a short focal length end to a long focal length end.
In the above-mentioned reference documents, due to not using an optical material having a high refractive index to produce the negative lenses of the second group optical system, manufacturing error sensitivity of the negative lens can not be controlled.
In addition, there is also a drawback in JP2005-24988A in that miniaturization is not sufficiently attained.